unison/unison-src/new-runtime-transcripts/mvar.output.md

# tests for io2.MVar

`MVar`s are mutable, sharable storage for a single value, which may or
may not be present at any given time. It is sharable in the sense that
it is safe for multiple threads to attempt simultaneous reading and
writing to and from the same MVar safely.

MVars are the building block on which many other concurrency
primitives can be built, such as Futures, Run at most once initializer
blocks, Queues, etc.


```unison
testMvars: '{io2.IO}[Result]
testMvars _ =
  test = 'let
    test = "test"
    test2 = "test2"
    ma = MVar.new test
    check "ma should not be empty" (not (isEmpty ma))
    test' = take ma
    expectU "should reap what you sow" test test'
    check "ma should be empty" (isEmpty ma)
    put ma test
    test'' = swap ma test2
    expectU "swap returns old contents" test test''
    test''' = swap ma test
    expectU "swap returns old contents" test2 test'''

    ma2 = !MVar.newEmpty
    check "tryTake should succeed when not empty" (not (isNone (tryTake ma)))
    check "tryTake should not succeed when empty" (isNone (tryTake ma))

    check "ma2 should be empty" (isEmpty ma2)
    check "tryTake should fail when empty" (isNone (tryTake ma2))


  runTest test
```

```ucm

  I found and typechecked these definitions in scratch.u. If you
  do an `add` or `update`, here's how your codebase would
  change:
  
    ⍟ These new definitions are ok to `add`:
    
      testMvars : '{io2.IO} [Result]

```
```ucm
.> add

  ⍟ I've added these definitions:
  
    testMvars : '{io2.IO} [Result]

.> io.test testMvars

    New test results:
  
  ◉ testMvars   ma should not be empty
  ◉ testMvars   should reap what you sow
  ◉ testMvars   ma should be empty
  ◉ testMvars   swap returns old contents
  ◉ testMvars   swap returns old contents
  ◉ testMvars   tryTake should succeed when not empty
  ◉ testMvars   tryTake should not succeed when empty
  ◉ testMvars   ma2 should be empty
  ◉ testMvars   tryTake should fail when empty
  
  ✅ 9 test(s) passing
  
  Tip: Use view testMvars to view the source of a test.

```